The yeast Saccharomyces cerivisiae is the brewer's go-to fungus for converting sugar into alcohol and carbon dioxide in potent potables like ciders and ales, but when it comes to brewing lager beer... well, lager brewing is a completely different animal. In fact, to be more specific, lager-brewing requires a different organism altogether.

The modern-day lager-brewing yeast goes by the name of Saccharomyces pastorianus, but S. pastorianus is a fully domesticated species, the descendant of a long-lost strain of yeast that scientists believe led to the accidental discovery of lager beer centuries ago. For decades, scientists have scoured the globe for traces of this borderline mythical microbe — and now it would appear their search is finally over.

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Everyone, meet Saccharomyces eubayanus. Actually, the orange, protuberant, extraterrestrial-looking thing you're looking at is called a gall, and it's growing on a beech tree somewhere deep in the frozen forests of Patagonia in South America. But the insides of these galls are teeming with S. eubayanus, the wild strain of yeast that an international team of scientists believes was instrumental in bringing about the accidental invention of lager-brewing in Bavaria some 600 years ago.

"People have been hunting for this thing for decades," said University of Wisconsin-Madison genetics professor Chris Todd Hittinger, who co-authored a study — published in the latest Proceedings of the National Academy of Sciences — documenting the yeast's discovery.

"And now we've found it. It is clearly the missing species. The only thing we can't say is if it also exists elsewhere (in the wild) and hasn't been found."

Lager-brewing is believed to have developed accidentally when Bavarian monks, who traditionally brewed ales with the yeast strain S. cerevisiae, decided to store their beer barrels in caves.

Because lager-brewing yeasts tend to prefer cooler temperatures (daily lows in the Patagonian forest that S. eubayanus was found in are around -2C), the scientists believe that after somehow finding its way to Europe, the cooler conditions of the brewing caves likely allowed S. eubayanusto out-compete and hybridize with S. cerevisiae, giving rise to a lager beer in the process.

"The hybrid almost definitely formed accidentally and people adopted it because the beer came out differently," Hittinger said.

The researchers' hypothesis is supported by substantial genetic evidence. When the team analyzed the Patagonian yeast's genome, they found its genetic code was almost identical to that of the modern-day lager-brewing hybrid S. pastorianus.

"It proved to be distinct from every known wild species of yeast, but was 99.5 percent identical to the non-ale yeast portion of the lager genome," Hittinger said.